Solve for $t$ : $6 + t = 15$
Subtract $6$ from both sides: $ \begin{eqnarray} \\ 6 + t &=& 15 \\ \\ {-6} && {-6} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{6 + t} &\hphantom{=}& \hphantom{15} \\ t &=& 15 {- 6} \\ \end{eqnarray} $ Simplify. $ t = 9$